LNK1 and LNK2 Corepressors Interact with the MYB3 Transcription Factor in Phenylpropanoid Biosynthesis.

Subgroup 4 of R2R3-MYB transcription factors consists of four members, MYB3, MYB4, MYB7, and MYB32, which possess the conserved EAR repression motif (pdLHLD/LLxiG/S) in their C termini. Here, we show that MYB3 is a newly identified repressor in Arabidopsis (Arabidopsis thaliana) phenylpropanoid biosynthesis. However, the repression mechanism of MYB3 is completely different from MYB4, MYB7, and MYB32. Yeast two-hybrid screening using MYB3 as a bait isolates NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED1 (LNK1) and LNK2, members of a small family of four LNK proteins. The repression activity of MYB3 to cinnamate 4-hydroxylase (C4H) gene expression is directly regulated by corepressors LNK1 and LNK2, which could facilitate binding of MYB3 with C4H promoter. The two conserved Asp residues in both region 1 and 2 domain of LNKs are essential to mediate protein-protein interaction. Importantly, the Extra N-terminal Tail domain plays a negative role in LNK-MYB3 transcription complex-dependent repression of the C4H gene. We conclude that LNK1 and LNK2 act as transcriptional corepressors necessary for expression of the phenylpropanoids biosynthesis gene C4H through recruitment to its promoter via interaction with MYB3.